Currently, cellular radio frequency ("RF") plans generally assign radio channels, or frequencies, to cells within an RF network based on forecasts and ongoing studies of traffic patterns. Unfortunately, this scheme, referred to as "Fixed Channel Allocation" or "FCA", does not take into account the dynamic nature of traffic in certain portions of a coverage area. For example, in a high-density business corridor, the channel requirements in a subcell A during a given time period may be higher than the capacity of subcell A and the result in call blockage. At the same time, a neighboring subcell B may have idle channels due to less-than-capacity call rates.
One solution to this problem is to increase the number of channels assigned to subcell A to handle the higher call volumes at the expense of other cells and possibly violating existing reuse patterns. However, this does not account for the mobile nature of traffic, because at another period in time, subcell B may be experiencing call blockage, while channels in subcell A remain idle.
Another problem with FCA is that cell sites are shrinking as demand for capacity increases. Due to both irregularities in propagation and traffic distribution in these small cells, pre-assignment of channels becomes difficult.
Implementation of a centralized intelligence to monitor channel usage in various cells and assigning channels based on need is not practical, due to the large amount of overhead processing and messaging necessary to keep both the centralized intelligence and the individual cells apprised of channel usage. Additionally, inter-mobile telephone exchange ("MTX") messaging would involve changes to IS-41 messaging.
A number of papers have been published describing a scheme known as "Adaptive Channel Allocation" or "ACA". Most of these proposals deal with "Dynamic Channel Assignment" ("DCA"), which comprises the ability to dynamically allocate the entire spectrum based on need, without any frequency scanning per cell.
Other relevant channel assignment schemes include "Hybrid Channel Assignment" ("HCA") and "Borrowing Channel Assignment" ("BCA"). In HCA, a combination of FCA and DCA, a portion of the total frequency channels uses FCA and the rest use DCA. In BCA, when all the fixed channels of a cell ("acceptor cell") are occupied, the acceptor cell borrows free channels from a neighboring cell (donor cell). In generally, BCAs use some form of central control to lock out other cells from using the channel(s) borrowed by the acceptor cell. While BCA is effective under light to moderate traffic conditions, especially when compared to FCA, under heavy traffic, channel borrowing may be high enough to cause channel usage efficiency to drop drastically and increase blocking probability due to channel locking.
Segregation is described in the literature as a self-organizing dynamic channel allocation scheme. Channels are assigned probabilities of being clear based upon successful use of that channel for a call. When a call arrives at a cell, the determination of which channel to use to service the call is based on the current probabilities of the channels. Carriers sense (using received signal strength measurement) is performed on the selected channel (i.e., the channel with the highest probability) to ensure that it is clear to use. If not, the probability of that channel is decreased and the next channel is tried. If that channel is clear, the call is assigned to that channel and its probability is increased.
While segregation offers certain advantages over previously-described methods, it, too, suffers from certain deficiencies, including the fact that the time for convergence to optimal allocation is very long. In addition, the determination of carrier sense only at channel request is shown to cause extensive delay and if delay is limited to a finite wait time, performance of the system degrades rapidly.
Therefore, what is needed is an improved system and method of allocating radio frequency channels in a cellular network environment which take advantage of the best qualities of FCA and distributed channel borrowing utilizing a segregation scheme.